In the field of bipolar and BICMOS integrated circuits, junction diodes are used for a variety of well-known purposes. One such diode is the Schottky-type; that is, a junction diode comprising a metal in contact with the surface of a semiconductor material. Prior art Schottky devices of integrated circuits are typically formed by depositing the metal on the surface of an epitaxial layer of semiconductor material. The epitaxial layer is formed of single-crystal, N type semiconductor material deposited on a substrate of P type semiconductor material. In prior art devices this epitaxial layer is doped uniformly through the bulk, and has a surface concentration of about 1-2.times.10.sup.16 donors/cc, resulting in a resistivity of about 0.5-2.0 ohm-cm at the metal-semiconductor interface. At this concentration the sheet resistance of the epitaxial layer is about 3400 ohms/sq and the required turn-on voltage, or the barrier height, for the Schottky diode is about 0.45 V @ 30 nA/sq. micron. This surface concentration yields a good low-reverse leakage diode. However, a number of problems exist with the prior art Schottky diodes for integrated circuits, principally related to high series resistance and wide variations in donor concentration along the current path under the active diode from one diode to the next.
As illustrated in FIG. 1, a cross-sectional view of a typical prior art Schottky diode configuration, it can be seen that the current path from the diode 1 to a contact 2 proceeds directly through the epitaxial layer 3. This epitaxial layer is doped with donor concentrations previously indicated by, for example, incorporating those donors into the deposition material, as has been described by Peltzer in U.S. Pat. No. 3,648,125. The donor concentration is essentially uniform at all depths for a given deposition sequence. While the 1-2.times.10.sup.16 donors/cc surface concentration results in acceptable reverse current leakage and turn-on voltage characteristics, that same concentration throughout the bulk results in a relatively high pathway series resistance. This high series resistance means a slower-operating Schottky diode.
Another problem associated with prior art Schottky diode fabrication methods is a variation in donor concentration from one device to the next. For a number of reasons, donor concentrations can vary by as much as .+-.30% from one fabrication sequence to the next. Actual donor concentrations in the epitaxial layer are not known until after the deposition process has occurred. This fluctuation translates into performance variability for a given lot of fabricated diodes. This variability relates to the forward voltage drop and the reverse current leakage at the metal-semiconductor interface for the particular diode, in addition to the series resistance through the bulk of the epitaxial layer. That is, the forward voltage drop value, the reverse current leakage rate and the series resistance may vary from one diode to the next by as much as .+-.30%.
Recent innovations in integrated circuit bipolar NPN transistor structure fabrication methods have led to increasing localization and miniaturization of active areas. See, for example, co-pending application Ser. No. 07/655,676, of Robinson et al., filed Feb. 14, 1991, entitled Bipolar Transistor Structure and BICMOS IC Fabrication Process. Disclosed in that application is a new bipolar transistor structure and fabrication method that includes the incorporation of bipolar and CMOS fabrication steps. The process disclosed involves implanting relatively slow-diffusing N type atoms in the P type substrate so as to form a buried N.sup.+ collector layer for the bipolar structure, using a conventional collector mask, etch and implant sequence. Next, a sub-emitter collector region of relatively fast-diffusing N type atoms in an N.sup.++ concentration is implanted into a portion of the buried collector layer. At the same time, the same concentration of the same N type atoms is implanted in another portion of the P type substrate to form a retro NWell for a CMOS transistor structure. The simultaneous implant is provided by means of a retro NWell and sub-emitter collector mask, etch and implant sequence. The epitaxial layer of semiconductor material, with an N.sup.- concentration of donors, is then deposited on the substrate. This concentration, typically about 2.times.lO.sup.15 donors/cc, is made much less than N type atom concentrations in prior art epitaxial layers in order to minimize parasitic collector/base capacitance in the new bipolar structure and to localize sub-elements of the BICMOS integrated circuit.
After epitaxial layer deposition, subsequent thermal cycling causes the sub-emitter collector layer and the retro NWell of the CMOS structure to up-diffuse, or "retrograde diffuse" through the epitaxial layer by a well-defined pathway. This retrograde diffusion yields a donor concentration gradient through these localized areas of the epitaxial layer. Near the epitaxial layer surface the donor concentration is lowest; near the epitaxial layer/substrate interface the concentration is greatest.
Isolation regions of silicon dioxide are formed simultaneously in both the bipolar and CMOS transistor structures using a conventional active area mask, etch and growth sequence, to provide surface isolation between active areas. In the final stages of bipolar and CMOS transistor structure fabrication, contact deposition regions are formed through a contact definition mask, etch and deposition sequence, using a passivating silicon nitride layer as the mask. A first metal is then deposited in the contact regions, and a second metal layer is deposited over the first metal layer, wherein an interlayer dielectric may be formed so as to separate the two layers.
A disadvantage of the described prior art structure is that the relatively lightly-doped epitaxial layer is unsuitable for a Schottky diode. Specifically, a diode formed utilizing that substrate has a much higher turn-on voltage and higher reverse current leakage, due to the lower concentration of donors. In addition, the series resistance of the pathway through the epitaxial bulk is much higher for the same reason. Furthermore, the use of the low donor-concentration epitaxial layer as the Schottky diode substrate would still result in performance variability problems from one diode to the next because the deposition process remains the same and only the donor concentration is lower. Concentration variations as much as .+-.30% can still occur and with that diode performance characteristics can vary as well.
Therefore, what is needed is an improved Schottky diode with good low-reverse leakage and a relatively low turn-on voltage. What is also needed is a Schottky diode that has a relatively low-resistance current path through the bulk of the semiconductor material. Further, what is needed is a Schottky diode that is applicable for bipolar and BICMOS integrated circuits. Still further, what is needed is a new BICMOS fabrication process that includes the fabrication of the new Schottky diode using BICMOS mask sequences modified to include the formation of the new Schottky diode. Furthermore, what is needed is a Schottky diode fabrication process incorporated into a BICMOS fabrication process that results in reduced diode characteristic variability from one diode to another. Therefore, what is needed is to provide new mask structures and new mask, etch, implant and oxidation sequences for fabricating the new Schottky diode coupled to a BICMOS transistor structure without increasing the number of BICMOS processing steps.